1. Field of the Invention
The present invention relates in general to a powder metallurgy technology for producing sintered articles, and more particularly to a method of producing a sintered metal sprocket and a sprocket produced by the method, which is suitable for a silent chain used in an internal combustion engine or the like. More specifically, the present invention is concerned with a method of producing a sintered metal sprocket which has excellent wear and abrasion resistance.
2. Description of the Prior Art
As one of methods of increasing mechanical strength of sintered metal parts, Laid-Open Japanese Patent Publication 7-112231 shows a technique for increasing the strength of a sintered metal gear. That is, in the technique, the crystal grains gathering near each tooth of the gear are made minute and the mechanical strength of each tooth, particularly, the fatigue strength of the root of each tooth is increased. That is, in the method, after being heated to an austenitizing range, the peripheral portion of a circular blank of the gear is subjected to a hot form rolling to form teeth around the blank. Furthermore, as the need arises, the circular blank is subjected to a preparatory hot form rolling, before the major hot rolling, for obtaining much minute formation of the crystal grains or after the hot form rolling, the gear is subjected to an induction hardening for obtaining much hardened teeth of the gear.
The method described in the above-mentioned publication makes no mention of the improvement in wear and abrasion resistance of the teeth of the sintered gear. In other words, the technique of the publication can not be directly applied to a method for producing a sintered metal sprocket for a silent chain where high wear and abrasion resistance of the teeth of the sprocket is highly needed.
Hitherto, meshing between a sintered metal sprocket and a silent chain has been analyzed. By this analyzation, it has been revealed that a slippage between the sprocket and the silent chain is caused by a sliding movement of each unit of the link plates of the chain in a direction from the addendum to the dedendum of the sprocket upon receiving a certain impact. Furthermore, it has been also revealed that during the sliding movement of the link plates, side edges of each unit of the link plates is forced to make a point contact with each tooth of the sprocket against a bearing stress of over 0.4 Gpa. Furthermore, by making a detailed observation on the abrasion of the teeth of sintered metal sprockets, it has been revealed that as service duration increases, minute holes in the sintered body become merged thereby triggering a so-called peeling abrasion of the teeth of the sprocket. This abrasion manner is considered quite unique differing from that occurring in a sprocket for a roller chain or when two gears mesh each other.
In view of the above-mentioned revelation and consideration, the applicants have concluded that a satisfied wear and abrasion resistance of the teeth of sintered metal sprockets is not obtained by only fining the crystal grains. Furthermore, the applicants have revealed that an ordinary hot form rolling does not provide a deterrent against the abrasion of the teeth because the ordinary form rolling fails to bring about metallurgical connection between the crystal grains, and revealed that the induction hardening after hot form rolling does not provide a deterrent against the peeling abrasion of the teeth.
Thus, an object of the present invention is to provide a sintered metal sprocket for a silent chain and a method of producing the same, which are provided by embodying the above-mentioned revelation and consideration.
According to the present invention, there is provided a sintered metal sprocket for a silent chain, whose tooth portion shows a satisfied performance in mechanical strength, particularly in wear and abrasion resistance.
According to the present invention, there is further provided a method for producing a sintered metal sprocket for a silent chain, that has such a satisfied mechanical strength.
According to a first aspect of the present invention, there is provided a method of producing a sintered metal sprocket for a silent chain. The method comprises preparing a base mixture which contains a metallurgical iron powder, a lubricant and a graphite powder; subjecting the base mixture to a compression molding while heating the same above approximately 100xc2x0 C. thereby to produce a green compact; and sintering the green compact at a temperature above approximately 1180xc2x0 C. to produce the sintered metal sprocket.
According to a second aspect of the present invention, there is provided a method of producing a sintered metal sprocket for a silent chain. The method comprises preparing a base mixture which contains a metallurgical iron powder, a lubricant and a graphite powder, the metallurgical iron powder containing in weight ratio 2.0% to 5.0% of Ni (nickel), 0.2% to 1.0% of Mo (molybdenum), 0.5% to 2.0% of Cu (copper), 1% of C (carbon) and residue of Fe (iron) and impurities, the metal grains of Ni, Mo and Cu being diffusion-bonded onto each grain of Fe; subjecting the base mixture to a compression molding while heating the same above approximately 100xc2x0 C. thereby to produce a green compact of sprocket; and sintering the green compact of sprocket at a temperature above approximately 1180xc2x0 C. to produce the sintered metal sprocket.
According to a third aspect of the present invention, there is provided a method of producing a sintered metal sprocket for a silent chain. The method comprises preparing a base mixture which contains a metallurgical iron powder, a lubricant and a graphite powder, metallurigical iron powder containing in weight ratio 0.5% to 3.0% of Ni, 0.5% to 2.0% of Mo, 1% of C and residue of Fe and impurities, the metal grains of Ni being diffusion-bonded to each alloyed grain of Fe and Mo; subjecting the base mixture to a compression molding while heating the same above approximately 100xc2x0 C. thereby to produce a green compact of sprocket; and sintering the green compact of sprocket at a temperature above approximately 1180xc2x0 C. to produce the sintered metal sprocket.